That is a player-written custom note, though. [...] Maybe you forgot about writing it...
This. You must have written that note and forgot you wrote it, because that is exactly what a player-made note looks like that. It even shows on the map that you made a note on that zone. You just forgot that you made it.
While we're on the subject of doors that can't be opened, are there any plans for us to access a certain corridor in a certain end-game location? A part of my brain is still convinced that if I keep pressing random keys eventually the gate will open.
[joking]Maybe that's where the super-secret Siphoner Man superboss is located???[/joking]